1. Technical Field
The present invention relates to a scanning-type image display device.
2. Related Art
In recent years, a scanning-type image display device has been proposed that displays images by raster scanning a beam of light such as laser light on a projection surface.
Since this device can achieve a perfect black by stopping the supply of laser light, a higher contrast display is possible compared to a projector or the like that uses a liquid crystal light valve, for example.
Moreover, since an image display device that employs laser light is characterized by having a high color purity due to the laser light being a single wavelength and by readily shaping (focusing) the beam due to high coherence, it is expected to be used as a high-definition display that realizes high resolution and high color reproduction.
Also, in contrast to liquid crystal displays and plasma displays or the like, since the scanning-type image display device does not have fixed pixels, there is no concept of pixel number and so has the advantage of easily changing the resolution.
In order to generate an image with the scanning-type image display device, it is necessary to scan light in two dimensions using scanners such as a polygon mirror or a galvano-mirror.
Although there is a method of scanning light in two dimensions while moving a single scanner in the two directions of a horizontal direction and a vertical direction, in this case, there is a problem in that the constitution and control of the scanning system becomes complicated.
Therefore, there has been proposed a scanning-type image display device provided with two sets of scanners each scanning light in one dimension so as to incorporate horizontal scanning and vertical scanning, respectively.
Conventionally, it is normal to use a polygon mirror or a galvano-mirror for a scanner of the both directions. A projection device that employs a polygon mirror in scanner of the both directions is disclosed in Japanese Unexamined Patent Application No. H01-245780.
Although a device that uses a polygon mirror is introduced in Japanese Unexamined Patent Application No. H01-245780, increasing the resolution of an image format entails higher scanning frequencies, which approaches the limits of a polygon mirror or a galvano-mirror.
Therefore, in recent years systems have been announced that utilize Micro Electro Mechanical Systems (MEMS) technology in high-speed scanners.
A scanner that utilizes MEMS technology (hereinafter simply referred to as a “MEMS scanner”) is one manufactured utilizing micro-processing technology of a semiconductor material, such as silicon, and drives a mirror supported by a torsion spring or the like with electrostatic force or the like.
This scanner can scan light by causing the mirror to travel in a reciprocating motion via the interaction of the electrostatic force and the restoring force of the spring.
By using a MEMS scanner, it is possible to achieve a scanner that can be operated at a high frequency and large deflection angle compared to a conventional scanner. Thereby, it is possible to display high-resolution images.
In order to realize scanning with a high speed MEMS scanner, since the mirror must be made to travel in a reciprocation manner at the resonance point, assuming a given resolution, it is necessary to design and fabricate the resonance point of the mirror so as to agree with that resolution.
However, in the case that the resonance frequency of the mirror cannot be made to be a common multiple of the refresh rate of a display image, by driving the mirror deviated from the resonance point, it is impossible to ensure sufficient scanning amplitude.
Conversely, if the scanning frequency is matched with the resonance frequency, synchronization of the horizontal scanning and vertical scanning cannot be achieved, leading to image distortion.
Moreover, even if a mirror is manufactured so that the resonance frequency of the mirror is perfectly adjusted to common multiple of the refresh rate, it is conceivable for the temperature of the MEMS mirror to change as a result of the usage environment and the illumination state of the light, which would thereby cause slight change in the resonance point of the mirror.
In this case as well, as stated above, synchronization of the horizontal scanning and vertical scanning cannot be achieved, leading to image distortion.
Also, in the case of manufacturing errors in the above-mentioned scanner and changes in the resonance point during use or the like, synchronization of the image signal and the scanner cannot be achieved, leading to the generation of frames that cannot be displayed.
Examples of problems in the case of using a MEMS mirror were given above, but these problems are not limited to a device using a MEMS mirror, being common to devices that use other resonant-type scanners such as a resonant-type galvano-mirror or the like.